While currently hypothetical constructs, if you could spin two wormholes at extremely different speeds, accelerating one into a lower orbit while the other maintaining a slower orbit, one end of the wormhole would be experiencing time slower then the other.
Essentially you created a bridge between two points in time, going further and further apart - when you experience 3:00am, the other wormhole's "clock" would be on 2:59am, going through them would be going 1 minute into the past. the fast spinning wormhole would never be able to go backwards in time, so the furthest back point would still be going forward in time, just at a slower rate. You would certainly not be able to use this to travel to before this gate system was created. But if you continue to accelerate and lower the orbit of the "past" wormhole, you can reach a considerable difference (it would help if you had a very heavy gravity well to be able to hold it at high speeds without it going runaway). I suspect that Since the timelines that where altered never existed, we'd never actually experience going back in time, we would just start getting a ton of shit coming through from futures that never get to happen.